A wind-driving disc model for the mm-wavelength polarization structure of HL Tau

TL;DR

This paper proposes a wind-driving disc model to explain the polarization structure of HL Tau, showing that radial magnetic fields dominate in the polarized emission region, which supports the presence of centrifugally driven winds.

Contribution

It introduces a wind-driven disc model with a dominant radial magnetic field component to better fit polarization data of HL Tau, challenging previous interpretations of magnetic field configurations.

Findings

Radial magnetic fields better fit polarization data of HL Tau.

Small grains above the mid-plane produce high polarization.

Large grains settled in the mid-plane produce low polarization.

Abstract

The recent advent of spatially resolved mm- and cm-wavelength polarimetry in protostellar accretion discs could help clarify the role of magnetic fields in the angular momentum transport in these systems. The best case to date is that of HL~Tau, where the inability to produce a good fit to the 1.25-mm data with a combination of vertical and azimuthal magnetic field components was interpreted as implying that centrifugally driven winds (CDWs) are probably not a significant transport mechanism on the $\sim 10^2\,$au scale probed by the observations. Using synthetic polarization maps of heuristic single-field-component discs and of a post-processed simulation of a wind-driving disc, we demonstrate that a much better fit to the data can be obtained if the radial field component, a hallmark of the CDW mechanism, dominates in the polarized emission region. A similar inference was previously made in modelling the far-infrared polarization map of the pc-scale dust ring in the Galactic centre. To reconcile this interpretation with theoretical models of protostellar discs, which indicate that the wind is launched from a comparatively high elevation above the mid-plane, we propose that most of the polarized emission originates -- with a high ($\ga 10$\%) intrinsic degree of polarization -- in small ($\la 0.1\,$mm) grains that remain suspended above the mid-plane, and that the bulk of the mm-wavelength emission is produced -- with low intrinsic polarization -- by larger grains that have settled to the mid-plane.